The bipolar junction transistor (BJT) is a well known and frequently used semiconductor electronic device. A bipolar junction transistor is generally defined as a device formed of a semiconductor material having two p-n junctions in close proximity to one another. In operation, current enters a region of the semiconductor material adjacent one of the p-n junctions which is called the emitter. Current exits the device from a region of the semiconductor material adjacent the other p-n junction which is called the collector. Generally speaking, the collector and emitter will have the same conductivity type, either p or n. A small portion of semiconductor material having the opposite conductivity type (either p or n) from the collector and the emitter is positioned between the collector and the emitter which is known as the base. The transistor's two p-n junctions exist where the collector meets the base and where the base meets the emitter. Because of their respective structures and conductivity types, bipolar junction transistors are generally referred to as n-p-n or p-n-p transistors.
In operation, when current is injected into or extracted from the base, depending upon whether the transistor is n-p-n or p-n-p, , it will greatly affect the flow of charge carriers (i.e. electrons or holes) which can move from the collector to the emitter. Typically, small currents applied to the base can control proportionally large currents passing through the transistor, giving it its usefulness as a component of electronic circuits.
One of the requirements for an operable and useful junction transistor is an appropriate semiconductor material from which it can be formed. The most commonly used semiconductor material is silicon (Si), and recently attention has grown with respect to other semiconductor materials such as gallium arsenide (GaAs) and indium phosphide (InP). For given circumstances and operations, these materials all have appropriate applications.
Another material for which great semiconductor potential has long been recognized, but for which appropriate techniques for producing both the material itself and devices formed from it have been lacking, is silicon carbide (SiC). Silicon carbide has well-known advantageous semiconductor characteristics: a wide bandgap, a high thermal conductivity, a high melting point, a high electric field breakdown strength, a low dilectric constant, and a high saturated electron drift velocity. Taken together, these qualities mean that electronic devices formed from silicon carbide should have the capability of operating at higher temperatures, at high device densities, at high speeds, at high power levels and even under high radiation densities, as compared to other semiconductor materials. Accordingly, attempts to produce device quality silicon carbide and devices formed from silicon carbide have been of interest to scientists and engineers for several decades.
Silicon carbide is, however, a difficult material to work with. It crystallizes in well over 150 polytypes, some separated by very small thermodynamic differences. As a result, from a practical standpoint, growth of the sufficiently defect-free single crystals required for electronic devices has remained an elusive goal. Silicon carbide's high melting point renders techniques such as alloying or diffusion of dopants more difficult, usually because a number of the other materials necessary to perform such operations tend to break down at the high temperatures required to affect silicon carbide. Silicon carbide is also an extremely hard material, and indeed its most common use is as an abrasive.
Accordingly, the patent and scientific literature is filled with attempts to grow larger single crystals of silicon carbide and thin films of monocrystalline silicon carbide, as well as attempts to produce junctions, diodes, transistors and other devices from this material.
Several prior patents describe attempts to produce junction transistors on silicon carbide. One such attempt is described by Hall in U.S. Pat. No. 2,918,396. Hall discusses a method of forming junction transistors from silicon carbide which emphasizes forming mechanical and electrical contacts. Hall's technique calls for placing an alloy in contact with a monocrystalline wafer of silicon carbide. The alloy is formed from silicon and a potential dopant material. By heating the wafer of silicon carbide while it is in contact with the alloy, the alloy potentially melts, dissolves a small portion of the silicon carbide wafer, and allows the dopant to permeate the silicon carbide. Following cooling, a doped portion of silicon carbide results. If the silicon carbide starting material is already doped in an opposite conductivity type, a p-n junction also results. By performing the same technique on opposite sides of the monocrystalline portion of silicon carbide, two p-n junctions result, forming the basis for a junction transistor.
The device as described in this patent, however, have failed to meet with any commercial success, most likely because of the lack of control inherent in the described doping technique, as well as the relatively large samples of silicon carbide which are required, making them unsuitable for modern miniaturized electronic devices such as integrated circuits.
In a later patent, No. 3,201,666, Hall discusses a method of forming non-rectifying contacts to silicon carbide, another fundamental technique in producing a useful electronic device. According to Hall, such a non-rectifying contact can be formed by joining a body of monocrystalline silicon carbide to a base member formed of molybdenum, tungsten or a molybdenum-tungsten alloy, with an intermediate layer of another alloy in between, the intermediate alloy being a eutectic alloy of silicon carbide and the base material. As is the case with the earlier described Hall technique, the techniques taught in the '666 patent have failed to meet with commercial success, very likely because of the size requirements called for by Hall. Specifically, because one goal for silicon carbide semiconductor devices is to have them remain operable at rather high temperatures (Hall suggests 1000.degree. C.), any contacts made with silicon carbide must be able to withstand the thermal expansion which the silicon carbide will undergo at these temperatures. Hall suggests that a relatively large contact is a key to dealing with these problems. As stated above, however, any significant size requirement for silicon carbide devices essentially renders them useless for modern electronic devices. Such considerations may not have been a great factor at the time of Hall's work in the late 1950's and early 196O's.
Wallace, U.S. Pat. No. 3,254,280, discusses a unipolar transistor formed from silicon carbide which is formed by doping an entire crystal of silicon carbide to give it a first conductivity type, and then doping the entire outer portion of the crystal silicon carbide to give it a second conductivity type. In the resulting structure, a second conductivity type exterior surrounds a core of the first conductivity type. Wallace's device has likewise failed to gain any commercial or practical acceptance, probably because of the inherent difficulty in obtaining appropriate samples of silicon carbide. As is known to those familiar with electronic devices, the production of silicon carbide suitable for such devices has remained a rather elusive goal. As a result, continued interest in silicon carbide and devices formed from it has waned among many researchers.
In the scientific literature, Von Muench et al. also describe attempts to form bipolar junction transistors in silicon carbide, Silicon Carbide Field Effect and Bipolar Transistors, Technical Digest of 1977 International Electronic Device Meeting, Institute of the Electronic Engineers, New York, 1977, p. 337. Von Muench fabricated a bipolar junction transistor from 6H alpha silicon carbide using a multiple Cl gas etching sequence to obtain contact to the emitter and base of an n-p-n structure. The collector contact was made to the back of the n-type 6H alpha silicon carbide substrate. As indicated on p. 337 of Von Muench's paper, however, his technique required diffusion doping in silicon carbide at temperatures in excess of 1900.degree. centigrade or generation of p-n junctions by epitaxial growth of a doped silicon carbide layer. In particular, von Muench's paper admits that "ion implantation in silicon carbide has not yet been studied in detail."
Finally, Von Muench obtained his silicon carbide crystals by the Lely sublimation process, the earliest established sublimation technique, but one which has never resulted in producing silicon carbide of a crystal quality sufficient to produce commercial devices.
Recently, however, a number of significant developments have been accomplished in several areas of silicon carbide technology. A number of these developments are the subject of co-pending patent applications assigned to the common assignee of the present invention, and the contents of which are incorporated herein by reference.
These include: Davis et al. "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,921, filed Oct. 26, 1987, now U.S. Pat No. 4,912,063; Davis et al.; "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,573, filed Oct. 26, 1987, now U.S. Pat. No. 4,912,064; Edmond et al.; "Implantation and Electrical Activation of Dopants into Monocrystalline Silicon Carbide," Ser. No. 07/113,561, filed Oct. 26, 1987, now abandoned; and Davis et al. "Sublimation of Silicon Carbide to Produce Large, Device Quality Single Crystals of Silicon Carbide," Ser. No. 07/113,565, filed Oct. 26, 1987, now U.S. Pat. No. 4,866,005. Taken together, these developments have resulted in the production of large, device quality single crystals of silicon carbide, successful epitaxial growth of both beta and alpha silicon carbide thin films on such silicon carbide crystals, a novel and successful technique for introducing and activating dopant ions into silicon carbide, and the potential for forming various devices.
Accordingly, it is an object of the invention to provide a bipolar junction transistor formed on silicon carbide which is formed from an epitaxial layer of silicon carbide having a first conductivity type upon a substrate of silicon carbide having the opposite conductivity type in which the emitter has the same conductivity type as the substrate, and is formed in the epitaxial layer by high temperature ion implantation of doping ions into the epitaxial layer. As used herein, the term "substrate" can refer to both single crystal substrates or monocrystalline epitaxial thin films.
It is another object of the invention to provide a method of forming such a bipolar junction transistor.
It is a further object of the invention to provide a planar bipolar junction transistor capable of operating at high temperatures and under conditions of high radiation density, which transistor has a heavily doped collector and heavily doped emitter of a first conductivity type, a base of opposite conductivity type formed within a portion of doped silicon carbide having the same conductivity type as the collector and the emitter, and ohmic contacts formed upon the base, the emitter and the collector.
It is another object of the invention to provide a mesa bipolar junction transistor capable of operating at temperatures over 500.degree. centigrade and under conditions of high radiation density which transistor comprises a collector and emitter formed of the same heavily doped conductivity type silicon carbide, a base formed of doped silicon carbide of the opposite conductivity type from the collector and the emitter, and respective ohmic contacts upon the base, the emitter, and the collector.
Other objects and features of the invention and the manner in which the same are accomplished, along with exemplary examples, will be better understood according to the detailed description of the invention, taken in conjunction with the accompanying drawings in which: